GEOLOGICAL SURVEY CIRCULAR 83 April 1951 GEOLOGY AND GROUND-WATER RESOURCES OF THE HELENA VALLEY, MONTANA By Howard W. Lorenz and Frank A. Swenson With a Section on the CHEMICAL QUALITY OF THE WATER By Herbert A. Swenson Prepared as Part of a Program <O* of the Department of the Interior for Development of the Missouri River Basin UNITED STATES DEPARTMENT OF THE INTERIOR Oscar L. Chapman, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director Washington, D. C. Free on application to the Geological Survey, Washington 25, D. C. GPO 82-800092 CONTESTS Page Abstract......................................................... 1 Introduction ..................................................... 2 Purpose and scope of investigation........................... 2 Location and extent of .rea.................................. 3 Previous investigations 3 Acknowledgments.............................................. 3 History of the area.......................................... 5 Well-numbering system........................................ 6 Geography........................................................ 7 Physiography................................................. 7 Drainage..................................................... 7 Climate ...................................................... 10 Agriculture and vegetation................................... 12 History of irrigation.........................o.............. 13 Mining....................................................... 1^ Transportation...............................o............... 15 Geology.......................................................... 15 Stratigraphy....<>............................................ 15 Geologic history............................ 0 ................ 18 Igneous rocks................................................ 19 Structure.................................................... 20 Faults and earthquakes................................... 20 Geologic formations and their water-bearing properties........... 21 Pre-Tertiary rocks........................................... 21 Springs.................................................. 2k Wells.................................................... 24 Tertiary "lake beds"......................................... 25 Quaternary deposits.......................................... 27 Source of ground-water recharge.................................. 27 Tertiary "lake beds"......................................... 27 Quaternary deposits.......................................... 28 Fluctuations of water levels........................c............ 28 Effects of earthquakes on ground water........................... 32 Chemical quality of water in the Helena Valley..............*.... ;32 Introduction..o»........................ 0 ... 0 ........,.....». .32 Mineral character of the ground water........................ ' 33 Quaternary deposits.............................. ^....... /£3 Tertiary "lake beds"..................................... 35 Mineral character of the surface water..........c............ 35 Relation of quality of water to recharge..<>......<,........... 37 Effects of waterlogging on the quality of the ground water... 37 Relation of quality of water to use.......................... 37 Summary of quality of water.................................. 39 Waterlogging in the Helena Valley................................ 39 Present conditions........................................... 39 Probable future conditions................................... io Summary and recommendations...................................... ^0 References....................................................... k2 Page Plate lo Geologic map of the Helena Valley, Mont............. In pocket 2. Section showing successive changes of water level In wells in Helena Valley, Mont., during June, July, August, and September, 19^8....................... 32 3o Map of the Helena Valley, Mont., showing location of wells, contour lines on the ground-water surface for April and August, 1948, and extent of water­ logged area 0 ,>.....»......<,......«. .... .. In pocket 4. Map of Helena Valley, Mont., showing location of wells sampled for quality-of-water analysis....... In pocket Figure 1. Map of the Missouri River "basin showing areas in which ground-water studies have been made under Missouri Basin program............................ k 2. Sketch showing we 11-numbering system used in this report............................................ 6 3. A, Northeast view of the Helena Valley showing the contrasting topography of the Belt Mountains in the background; B, View showing the contrasting topography of the valley and the folded and faulted beds south of Helena...................... 8 4. Precipitation records at Helena, Mont., l8&%-19^8... 11 5« A, Tertiary "lake beds" of Oligocene age; B, Aurif­ erous gravel below Last Chance Gulch.............. IT 6. North-south section across the Helena Valley north of Helena, Mont..........<>........................ 26 T« Hydrographs of shallow observation wells in fine­ grained material.................................. 29 8. Hydrographs of shallow observation wells in medium- grained material.................................. 30 9« Hydrographs of shallow observation wells in coarse­ grained material.......a.......................... 31 10. Graphical representation of the composition of Helena Valley waters.............................. 36 11. Classification of Helena Valley waters considered as irrigation supplies............................... 38 TABLES Page Table 1. Average monthly discharge of Tenmile Creek fro® 1936 to 19^5........................................... 9 2. Discharge measurements at two points on Prickly Pear *Creek, October 25, 19^. 0 ......................... 10 3. Sedimentary rocks of the Helena Valley and their water-bearing properties.......................... 22 4. Chemical analyses and related physical measurements of waters in the Helena Valley.................... 3lf 5. Records of water levels in wells in Helena Valley... ^3 6. Records of wells and springs in Helena Valley....... 63 GEOLOGY AND GROUND-WATER RESOURCES OF THE HELENA VALLEY, MONTANA By Howard W. Lorenz and Frank A. Swenson WITH A SECTION ON THE CHEMICAL QUALITY OF THE WATER By Herbert A. Swenson ABSTRACT A study of the geology and the ground-water conditions of the Helena Valley, in the west-central part of Montana, was made during the summer of 19*^80 The purpose of the study was to gather data on the occurrence of ground water and its relation to present and proposed irrigation in the area. With such data one may better predict the effects of the application of irrigation water to additional lands in the area. The Helena Valley is a small intermontane basin. It is surrounded by rocks ranging in age from pre-Cambrian to Cretaceous and its floor is under­ lain by Tertiary "lake beds," which are mantled by younger alluvial fans in much of the valley. Folded and faulted Paleozoic limestone, shale, and quartzite beds, more than 5>000 feet in total thickness, are exposed in the mountains south of Helena. These sedimentary rocks have been metamorphosed by intrusion of the adjacent Boulder batholith. Folded pre-Cambrian sedi­ mentary rocks form the other mountains that surround the area. The principal water-bearing formations are the thick alluvial fans and the underlying "lake beds." The water in the alluvial fans generally is under water-table conditions whereas the water in the "lake beds" in the lower part of the valley is confined under artesian pressure and produces large flows of good-quality water in places. Wells drilled into the deposits of coarse boulders, cobbles, and indurated tuffs of "lake beds" along the southern margin of the valley yield only meager supplies of water. The Cretaceous, Paleozoic, and pre-Cambrian rocks yield onlv small quanti­ ties of water to wells and springs. Approximately 8,500 acres of formerly productive farm land in the lower part of the Helena Valley is waterlogged at the present time as a result of the increased ground-water recharge caused by irrigation of the higher lands in the valley. If more lands in the higher part of the valley are irrigated, the extent of the waterlogged area will increase unless provision is made either to prevent additional ground-water recharge or to recover and dispose of the excess ground water. 2 GEOLOGY AND GROUND-WATER RESOURCES OF THE HELENA VALLEY, MONTANA Ground vaters in the valley contain moderate amounts of dissolved solids, the predominant ions being calcium, bicarbonate, and sulfate, Little difference is discernible in the composition of waters in the Qua­ ternary and Tertiary deposits; the dilute nature of these waters indicates that the valley fill is derived from formations containing only small amounts of soluble minerals. The quality of the ground water appears to be little affected by re­ charge of the ground-water reservoir from return irrigation flows. Water­ logged areas may in time show an increase in mineralization as a result of evaporation. Both ground and surface waters are classified as "excellent to good" on the basis of Wilcox's criteria for irrigation waters. Boron, percent sodium, and dissolved solids are relatively low. Waters used for drinking or domestic purposes are generally considered satisfactory except, in some instances, for hardness. INTRODUCTION Purpose and Scope of Investigation The purpose of this investigation was to gather data on the occur­ rence, conditions, and potentialities of the ground water in the